The 420-m-thick interval recovered at Site 1238 includes Quaternary-middle Miocene sediments. A well-constrained biostratigraphy is provided by the various microfossil groups (Table T9; Fig. F43). The section comprises a continuous 350-m-thick expanded sequence from the Pleistocene-uppermost Miocene (to ~6.6 Ma) and a ~75-m-thick condensed upper-middle Miocene sequence. A Miocene nannofossil assemblage, which includes Coccolithus miopelagicus and the absence of Cyclicargolithus floridanus in the deepest sample (202-1238A-46X-CC), suggests a basal age of 10.4-11.6 Ma. Planktonic foraminifers suggest a basal age of 11.68-13.18 Ma. This would result in extremely low sedimentation rates or a hiatus in the lowermost interval below ~420 mcd.
Calcareous nannofossils are generally abundant throughout the section, with good to moderate preservation except in the lower Pleistocene and middle Miocene (Cores 202-1238A-11H and 45X, respectively), in which nannofossils are rare and calcium carbonate contents are low (Fig. F25).
Planktonic and benthic foraminifers are common to abundant, except in two intervals (90-160 mcd and 330-460 mcd) where abundance decreases markedly. In contrast, radiolarians are abundant in these intervals. Preservation of foraminifers is good to moderate, except in the lower part of Hole 1238A. Benthic/planktonic foraminifer ratios generally increase downhole with ~1% benthic foraminifers down to 100 mcd, ~ 20% between 100 and 330 mcd, and ~50% or more below 330 mcd (Fig. F25).
Diatoms are common to abundant throughout the Pleistocene, Pliocene, and the upper half of the upper Miocene (~8-9 m.y.) (Fig. F25). Below Core 202-1238A-42X (~430 mcd), the lower half of the upper Miocene is barren of siliceous microfossils, except for rare fragments replaced by pyrite within the siliceous chalk in Cores 42X and 43X (~430-440 mcd).
Calcareous nannofossils are abundant and well to moderately preserved in all the samples examined (Table T10). Minor to moderate dissolution of nannofossils is evident where siliceous microfossils are highly abundant. Overgrowths on nannofossils are only minor. Presumably because of the high abundance of the siliceous microfossils, some traditional index nannofossil species, particularly some discoaster and ceratolith species, are rare or absent. However, discoaster abundance varies, probably indicating significant fluctuations in upwelling intensity.
The generally low abundance or the absence of some index species and the frequent reworking of nannofossils in the upper upper Miocene-lower Pliocene compromised the stratigraphic resolution to some extent. Nevertheless, a moderate-resolution nannofossil biostratigraphy has been established for the entire section. Nannofossil biostratigraphy suggests that the section spans a time interval from 0 to ~11 Ma and that the sequence is complete without any detectable hiatuses or sediment slumps.
Virtually all the well-known Pleistocene nannofossil events have been located within an uncertainty of ~1.5 m. These events are the beginning of the acme of Emiliania huxleyi (0.08 Ma), between Samples 202-1238A-1H-2, 75 cm (2.25 mcd), and 1H-3, 75 cm (3.75 mcd); the first occurrence (FO) of E. huxleyi (0.26 Ma), between Samples 2H-4, 75 cm (10.88 mcd), and 2H-5, 75 cm (12.39 mcd); the last occurrence (LO) of Pseudoemiliania lacunosa (0.46 Ma), between Samples 3H-CC, 14 cm (25.86 mcd), and 4H-1, 75 cm (30.05 mcd); the LO of Reticulofenestra asanoi (0.88 Ma), between Samples 6H-1, 75 cm (50.6 mcd), and 6H-2, 75 cm (52.11 mcd); the FO of R. asanoi (1.08 Ma), between Samples 7H-5, 75 cm (66.58 mcd), and 7H-6, 75 cm (68.09 mcd); the LO of large Gephyrocapsa spp. (1.24 Ma), between Samples 7H-6, 75 cm (68.09 mcd), and 7H-CC, 1 cm (68.95 mcd); the FO of large Gephyrocapsa spp. (1.45 Ma), between Samples 8H-5, 75 cm (76.79 mcd), and 8H-6, 75 cm (78.3 mcd); the LO of Calcidiscus macintyrei (1.59 Ma), between Samples 9H-3, 75 cm (81.68 mcd), and 9H-4, 75 cm (83.19 mcd); and the FO of medium Gephyrocapsa spp. (1.67 Ma), between Samples 11H-1, 75 cm (99.65 mcd), and 11H-2, 75 cm (101.2 mcd).
The youngest Pliocene nannofossil datum, the LO of Discoaster brouweri (1.96 Ma), is difficult to place precisely because of some reworking of the species into the Pleistocene. It is tentatively placed here between Samples 202-1238A-11H-6, 75 cm (107.2 mcd), and 11H-7, 40 cm (108.3 mcd), based mainly on the higher abundance of the species from the latter sample downhole. The LO of Discoaster pentaradiatus (2.44 Ma), between Samples 202-1238A-13H-3, 75 cm (124 mcd), and 13H-4, 75 cm (125.5 mcd), and the LO of Discoaster surculus (2.61 Ma), between Samples 13H-5, 75 cm (127 mcd), and 13H-6, 75 cm (128.6 mcd), also have some small uncertainties because of the relatively low abundance of discoasters in this part of the section. The LO of Reticulofenestra pseudoumbilicus (3.8 Ma) is placed between Samples 202-1238A-20H-CC (204.1 mcd) and 21H-CC (214.5 mbsf) based on the common occurrence of the species from the latter sample downhole. The generally rare presence of the species in a number of samples above this level are considered reworked because Sphenolithus abies is also very rare and P. lacunosa is found as deep as Sample 202-1238A-21H-CC.
The youngest Miocene datum, the LO of Discoaster quinqueramus (5.56 Ma), could not be determined at this site because of persistent reworking of nannofossils in the upper Miocene. This reworking problem can be seen by the presence of Ceratolithus sp. with D. quinqueramus in several samples in Cores 202-1238A-23X and 24X. The FO of the Ceratolithus genus is known to be at 5.0 Ma, and thus, the two species should not be present together. On the other hand, the very rare and sporadic Ceratolithus sp. does not allow a reliable use of its FO as a datum at the site.
A reliable late Miocene datum, the top of the R. pseudoumbilicus (>7 µm) absence interval (6.8 Ma), is located between Samples 202-1238A-38X-3, 75 cm (385.62 mcd), and 38X-6, 75 cm (390.2 mcd). The bottom of the R. pseudoumbilicus (>7 µm) absence interval (8.85 Ma) is placed between Samples 202-1238A-41X-4, 75 cm (418.96 mcd), and 41X-5, 75 cm (420.47 mcd). The oldest nannofossil datum determined at the site, the LO of C. miopelagicus (10.4 Ma), lies between Samples 202-1238A-45X-1, 75 cm (455.70 mcd), and 45X-2, 75 cm (457.20 mcd). The onset of siliceous sediment at the site at ~9 Ma (within Core 202-1238A-42X) perhaps signals a large change in wind-driven upwelling, caused by major uplift of the Andes or a change in regional oceanic nutrient budgets. The deepest sample (202-1238A-46X-CC) contains a Miocene nannofossil assemblage without C. floridanus. The latter species is generally ubiquitous in Miocene sediment older than 11.6 Ma. This suggests a basal age of 10.4-11.6 Ma at the site.
Planktonic foraminifers are generally abundant and well preserved in the upper part of Hole 1238A (mudline to Sample 202-1238A-9H-CC; 0-87.89 mcd). Below this depth, abundance and preservation vary markedly and the relative proportion of radiolarians and benthic foraminifers in the residue generally increases (Table T11; Fig. F25). Abundance decreases and preservation deteriorates significantly, particularly in two intervals between Samples 202-1238A-9H-CC and 15H-CC (87.89-152.15 mcd) and between Samples 37X-CC and 46X-CC (380.99-466.95 mcd). Near the base of Hole 1238A (Samples 202-1238A-43X-CC to 45X-CC; 437.44-466.95 mcd), recrystallized overgrowths frequently obscure test features.
Diversity is quite low overall, and assemblages tend to be dominated by upwelling taxa, such as Neogloboquadrina dutertrei or Neogloboquadrina pachyderma. Standard marker species are present throughout samples from the upper part of Hole 1238A and can be used to establish a preliminary biostratigraphy for the Pleistocene-Pliocene interval (Cores 202-1238A-1H to 17H; 5.01-173.29 mcd). However, few datums are available between Cores 202-1238A-18H and 41X (183.47-422.17 mcd) to interpret and constrain the biostratigraphy of the lower Pliocene-upper Miocene interval. The foraminiferal assemblage within this interval contains some blackened tests with mud infills that may indicate reworking. Two reliable datums between Samples 202-1238A-41X-CC and 46X-CC (422.17-466.95 mcd) indicate an age between 11.68 and 13.42 Ma for the base of Hole 1238A.
The well-preserved Pleistocene-late Pliocene planktonic foraminiferal assemblage includes Globigerina bulloides, Globigerina quinqueloba, Globigerinita glutinata, Globigerinoides ruber, Globorotalia menardii, Globorotalia scitula, Globorotalia tumida, N. dutertrei, N. pachyderma, Orbulina universa, and Sphaeroidinella dehiscens. The LO of G. ruber (pink) is placed between Samples 202-1238A-1H-CC and 2H-CC (5.01-15.56 mcd), and an age younger than 0.12 Ma can be assigned to the overlying section (upper Pleistocene Subzone Pt1b of Berggren et al., 1995) (Fig. F25; also see Fig. F12 in the "Explanatory Notes" chapter). The FO of G. ruber pink is recognized between Samples 202-1238A-2H-CC and 3H-CC (15.56-25.86 mcd), and an age between 0.12 and 0.40 Ma is attributed to the interval between the FO and LO of this species. An additional datum for Subzone Pt1b is provided by the FO of Globorotalia hirsuta (0.45 Ma) between Samples 202-1238A-3H-CC and 4H-CC (25.86-38.84 mcd).
The boundary between upper Pleistocene Subzone Pt1b and lower Pleistocene Subzone Pt1a is marked by the LO of Globorotalia tosaensis (0.65 Ma; Zone Pl5), which can be placed between Samples 202-1238A-4H-CC and 5H-CC (38.84-48.24 mcd). Three other useful datums indicating Subzone Pt1a are the LO of Globigerinoides obliquus (1.30 Ma), which is present between Samples 202-1238A-5H-CC and 6H-CC (48.24-59.78 mcd), and the LOs of Pulleniatina finalis (1.40 Ma) and Neogloboquadrina acostaensis (1.58 Ma), which can be identified between Samples 6H-CC and 7H-CC (59.78-68.95 mcd).
The Pliocene/Pleistocene boundary could not be recognized because of the absence of the standard zonal markers. However, the LO of P. finalis (2.04 Ma), between Samples 202-1238A-9H-CC and 10H-CC (87.89-98.20 mcd), indicates upper Pliocene Zone Pl6. The LO of Globorotalia pseudomiocenica (2.30 Ma), between Samples 202-1238A-10H-CC and 11H-CC (98.20-108.68 mcd), identifies the boundary between Zones Pl6 and Pl5. The LO of Globorotalia margaritae (3.58 Ma), between Samples 202-1238A-15H-CC and 16H-CC (152.15-162.10 mcd), marks the base of upper Pliocene Zone Pl3. Two useful markers for lower Pliocene Zone Pl2 are the LO of Pulleniatina primalis (3.65 Ma) and the LO of G. margaritae common (3.96 Ma), between Samples 202-1238A-16H-CC and 17H-CC (162.10-173.29 mcd). The FO of Globorotalia puncticulata (4.50 Ma), identified between Samples 202-1238A-28X-CC and 29X-CC (285.26-294.43 mcd), indicates Subzone Pl1b.
The Pliocene/Miocene boundary can be approximated by the FO of G. margaritae (6.09 Ma), between Samples 202-1238A-34H-CC and 35H-CC (349.48-359.96 mcd), which indicates uppermost Miocene Zone M14. The LO of Globorotalia fohsi s.l. (11.68 Ma), between Samples 202-1238A-41X-CC and 43X-CC (422.17-437.44 mcd), provides some useful age control in the lower part of Hole 1238A and points to much lower sedimentation rates or a hiatus in the lower upper Miocene and upper middle Miocene. The FOs of G. fohsi s.l. and Globorotalia robusta (13.42 and 13.18 Ma, respectively) cannot be identified, as both taxa are present in the lowermost sample recovered from Hole 1238A (Sample 202-1238A-46X-CC; 466.95 mcd).
The abundance and preservation of benthic foraminifers vary markedly in Hole 1238A (Table T11). The percentage of benthic foraminifers relative to total foraminifers is initially low (1% or less between the mudline and Sample 202-1238A-9H-CC; 0-87.89 mcd), then it fluctuates significantly between Cores 10X and 41X (98.20-422.17 mcd), reaching 99% in Sample 41X-CC (422.17 mcd). At depths below 422.17 mcd, benthic foraminifers represent only 1%-3% of the total foraminiferal assemblage. The Pleistocene-late Miocene assemblage is characterized by Cibicidoides mundulus, Globocassidulina subglobosa, Globobulimina affinis, Globobulimina pyrula, Gyroidinoides soldanii, Laticarinina pauperata, Melonis affinis, Melonis pompilioides, Oridorsalis umbonatus, Planulina wuellerstorfi, Pullenia bulloides, Pyrgo murrhina, Pyrgo serrata, Uvigerina peregrina, and Vulvulina spinosa.
Preliminary shipboard studies do not permit us to evaluate whether the changes in benthic foraminiferal abundance reflect variations in test accumulation rates (related to fluctuations in surface productivity) or stem from a preservation bias. However, changes in assemblage composition are apparent (although not quantified during shipboard study), particularly in the relative proportion of high productivity indicators such as Bulimina, Globobulimina, and Uvigerina, which suggest temporal changes in food resources at the seafloor, probably related to shifts in upwelling intensity and circulation patterns.
All core catcher samples from Hole 1238A were analyzed, as well as smear slides of some additional layers from the split cores. Diatom abundance and preservation vary from sample to sample, but diatoms are generally common to abundant (Fig. F25) and moderate to poorly preserved and very fragmented in the samples examined between Cores 202-1238A-1H and 41X. Sample 202-1238A-42X-CC contained chert, and the sequence below Core 43X through the bottom of the hole was barren of diatoms. Diatom assemblages in most samples vary from those dominated by Thalassiothrix and Thalassionema species to those dominated by Azpeitia nodulifer. Diatoms recovered from Site 1238 represent a continuous stratigraphic interval from the Holocene Fragilariopsis doliolus Zone to the late Miocene Nitzschia porteri Zone. Diatom biostratigraphy suggests that the sequence is complete with no detectable hiatuses (Table T12). However, the generally low abundance of some index species and the frequent reworking of older diatoms in the lower Pliocene-upper Miocene interval compromises the stratigraphic resolution to some extent and requires much more detailed shore-based work.
The mid-Pleistocene diatom datums, LOs of Nitzschia reinholdii (0.62 Ma) and of N. fossilis (0.70 Ma), are difficult to place precisely because of the rare and sporadic occurrences of those species. We have recognized the following reliable Pleistocene diatom datums. The LO of Rhizosolenia matuyamai (1.05 Ma) is placed between Samples 202-1238A-6H-4, 75 cm, and 6H-4, 100 cm (55.13-55.38 mcd); the FO of R. matuyamai (1.18 Ma) is recognized between Samples 202-1238A-7H-4, 75 cm, and 7H-5, 75 cm (65.07-66.58 mcd); the LO Rhizosolenia praebergonii var. robusta (1.73 Ma) is doubtful but may be placed between Samples 202-1238A-9H-2, 75 cm, and 9H-3, 75 cm (80.16-81.67 mcd); and the FO of F. doliolus is probably placed between Samples 202-1238B-11H-CC and 12-1, 75 cm (108.68-109.6 mcd).
The following late Pliocene diatom datums were recognized as follows: The LO of Thalassiosira convexa s.l. (2.41 Ma) is placed between Samples 202-1238A-12H-2, 75 cm, and 12H-3, 75 cm (111.11-112.6 mcd). The LO of Nitzschia jouseae (2.77 Ma) occurs between Samples 202-1238A-14H-4, 75 cm, and 14H-5, 75 cm (136.23-137.7 mcd). The FO of R. praebergonii s.l. (3.17 Ma) is placed between Samples 202-1238A-18H-2, 75 cm, and 18H-3, 75 cm (175.75-177.3 mcd). The FO of T. convexa var. convexa (3.81 Ma) occurs between Samples 202-1238A-22H-5, 75 cm, and 22H-CC (222.55-225.8 mcd).
The FO of N. jouseae (5.12 Ma) was recognized between Samples 202-1238A-29X-3, 75 cm, and 30X-3, 75 cm (290.73-301 mcd). The LO of Nitzschia cylindrica (4.88 Ma), is placed between Samples 202-1238A-28X-3, 75 cm, and 29X-3, 75 cm (279.82-290.4 mcd).
For the late Miocene, given the presence of reworked forms, the stratigraphic model is mainly based on FO datums but LOs have also been considered when in accordance with recognized FOs. The LO of Thalassiosira miocenica (5.84 Ma) was recognized between Samples 202-1238A-33X-3, 75 cm, and 34X-3, 75 cm (332.87-343.4 mcd). The LO of Nitzschia miocenica (6.08 Ma) was placed between Samples 202-1238A-34X-6, 75 cm, and 35X-1, 75 cm (347.95-351). The LO of Thalassiosira praeconvexa (6.18 Ma) was tentatively recognized between Samples 202-1238A-35X-CC and 36X-CC (359.76-370.5 mcd). The FO of T. miocenica (6.55 Ma) was placed between Samples 202-1238A-36X-CC and 37X-CC (370.29-380.9 mcd). The FO of T. convexa (6.57 Ma) is tentatively placed between Samples 202-1238A-36X-CC and 38X-CC (370.29-391.5 mcd). The LO of the Rossiela praepaleasea group (6.54 Ma) and the FO of T. praeconvexa (6.71 Ma) may be placed between Samples 202-1238A-37X-CC and 38X-CC (380.85-391.5 mcd). The FO of N. miocenica (7.30 Ma) is recognized between Samples 202-1238A-38X-CC and 39X-CC (391.51-402.3 mcd). The LO of Actinoclus ellipticus var. javanicus (7.79 Ma) is probably placed between Samples 202-1238A-40X-CC and 41X-CC (412.85-422.2 mcd).
The siliceous microfossil assemblage from Samples 202-1238A-41X-6, 75 cm, to 41X-CC (421.97-422.17 mcd), is characterized by the presence of the heavily silicified diatom Coscionodiscus marginatus, and the common presence of radiolarians. No age diagnostic diatom species have been observed, but the common presence of C. marginatus has been previously noted within the N. porteri Zone (7.30-8.23 Ma) and the upper portion of the Coscinodiscus plicatus Zone (equivalent to Thalassiosira yabei Zone; 8.23-9.76 Ma) (see Fig. F12 in the "Explanatory Notes" chapter) at Site 495 (Jousé et al., 1982).